A threaded joint has been popularly used to connect steel pipes used in a petroleum installation for oil industry such as oil country tubular goods. In connecting steel pipes used for search and production of oil or gas, conventionally, a standard threaded joint stipulated in API (American Petroleum Institute) standard has been typically used. However, recently, deepening of a well for crude oil or natural gas has progressed and the number of horizontal wells and directional wells from vertical wells has been increasing. Hence, the drilling and production environment has become difficult. Further, the number of wells developed in a severe circumstance such as oceans and polar regions has been increasing. Hence, the performance levels which threaded joints have to satisfy are diversified including compression resistance, bending resistance, and sealability against external pressure (external pressure resistance). In view of the above, the number of instances where a special threaded joint having high performances referred to as “premium joint” is being used has increased, and the demand to improve performances of the premium joint has been also steadily increased.
A premium joint is a coupling-type joint where externally-threaded members (hereinafter referred to as “pins”) each of which includes tapered threads, seal portions (to be more specific, metal to metal seal portions) and shoulders (to be more specific, torque shoulders) and are formed on pipe end portions, and an internally-threaded member which includes tapered threads, seal portions (to be more specific, metal to metal seal portions) and shoulders (to be more specific, torque shoulders) and connects the pins to each other (hereinafter referred to as “box”) are jointed to each other. The tapered threads are important to firmly fix the pipe joint, the seal portions play a role of ensuring gas tightness by bringing the box and the pins into metal contact at such portions, and the shoulders form shoulder faces which play the role of abutments during make-up of the joint.
FIGS. 4A-4C are schematic explanatory views showing a conventional example of an oil-well-pipe use premium joint. FIGS. 4A-4C are also longitudinal cross-sectional views of a threaded joint of a circular pipe (a cross-sectional view where a tube axis extends in the cross section). The threaded joint includes pins 3 and a box 1 corresponding to the pins 3. The pin 3 has an externally-threaded portion 7 and a nose (also referred to as a pin nose) 8 which is an elongated portion formed adjacent to the externally-threaded portion 7 on each distal end side of the pin 3 on an outer surface thereof and having no threads. The nose 8 has a seal portion (to be more specific, a metal to metal seal portion) 11 on an outer peripheral surface thereof, and a shoulder 12 on an end surface thereof. The box 1 corresponding to the pins 3 has internally-threaded portions 5, seal portions 13 and shoulders 14 on an inner surface thereof, and these portions are portions threadedly engaged with or brought into contact with the externally-threaded portions 7, the seal portions 11 and the shoulders 12 of the pins 3 respectively. In FIG. 4A, symbol AXIS indicates a tube axis, and tp0 indicates a pin raw pipe thickness.
In the conventional example shown in FIGS. 4A-4C, the threaded joint is of a type where the seal portion 11 is formed on a distal end portion of the pin 3, and desired sealability can be realized by imparting an appropriate make up torque. However, the make up torque is influenced by the lubrication condition, a surface quality and the like. As a design of a threaded joint which does not largely depend on these factors, there has been known a radial-directional seal method (also referred to as a “radial seal type”) where a radial directional component of a seal contact pressure is relatively increased.
With respect to the point that the seal portion is provided at a portion different from the shoulder, the radial-seal-type threaded joint is substantially equal to the threaded joint of a type having the seal portion on the distal end portion of the pin.
As a function which a threaded joint for an oil country tubular goods is required to possess besides a sealing property, corrosion resistance against crevice corrosion is named. Crevice corrosion occurs when a corrosive fluid infiltrates into a minute gap formed between the pin and the box, stagnates and condenses in the gap. In the threaded joint, particularly the gap between the shoulder faces is believed as being a problematic place where crevice corrosion is liable to occur.
As a means of overcoming the above-mentioned problem, it is known that a shoulder angle on a pin side is designed to be smaller than a shoulder angle on a box side by 0.5 to 4.0 degrees in advance by taking into consideration a difference in shoulder angle brought about by diametrically reduced deformation of a distal end of the pin due to a contact force of the seal portion thus making the shoulders uniformly contact with each other at the time of contact (see Japanese Unexamined Patent Application Publication No. 9-152067). According to such an approach, it is possible to prevent formation of a gap on an inner surface side or occurrence of the excessively large plastic deformation due to a non-uniform contact so that it is possible to acquire a threaded joint for an oil country tubular goods which can withstand being used some dozen times and prevents the occurrence of crevice corrosion. Further, the difference in shoulder angle may preferably be set in accordance with two parameters, that is, D/t which is a ratio between an outer diameter D of the pin or the box and a wall thickness t of the pin or the box and the difference δr (=δs−δt) between a fitting margin δs in a radial direction of a seal forming surface and a fitting margin δt in a radial direction of a thread forming surface.
However, the known methods cannot suppress plastic deformation of the whole pin distal end when a compressive load is applied, and a gap is generated between the shoulder faces due to the excessively large plastic deformation of front surfaces of the shoulders of the pin distal ends when a compressive load is applied to the area where crevice corrosion is liable to occur.